In the art of measuring mass flow rates of flowing substances it is known that flowing a fluid through a rotating or oscillating conduit induces Coriolis forces which act perpendicularly to both the velocity of the mass moving through the conduit and the angular velocity vector of the rotating or oscillating conduit. It is also known that the magnitudes of such Coriolis forces are related to both the mass flow rate passing through the conduit and the angular velocity of the conduit.
One of the major technical problems previously associated with efforts to design and make Coriolis mass flow rate instruments was the necessity either to measure accurately or control precisely the angular velocity of the conduit so that the magnitude of generated Coriolis forces could be determined and, therefrom, one could calculate the mass flow rate of the substance flowing through the conduit. Even if the angular velocity of the flow conduit could be determined or controlled, accurate determination of the magnitude of generated Coriolis forces was another technical problem previously associated with designing and making Coriolis mass flow rate instruments. This problem arises in part because the magnitude of generated Coriolis forces are very small, therefore resulting distortions of flow conduits which are oscillating or rotating are minute. Further, because of the small magnitude of the Coriolis forces, distortions of the conduit resulting from external sources such as invariably present vibrations induced, for example, by neighboring machinery or pressure surges in fluid lines cause erroneous determinations of mass flow rates. Such error sources may even completely mask the effects caused by generated Coriolis forces rendering the meter useless.
A mechanical configuration and measurement technique which, among other advantages: (a) avoids the need to measure or control the magnitude of the angular velocity of a Coriolis mass flow rate instrument's flow sensing conduit; (b) concurrently provides requisite sensitivity and accuracy for the measurement of effects caused by generated Coriolis forces; and, (c) is not susceptible to errors resulting from external vibration sources, is taught in U.S. Pat. Nos. Re. 31,450, 4,422,338 and 4,491,025. The mechanical configuration disclosed in these patents incorporates curved flow sensing conduits which have no pressure sensitive sections, such as bellows or other pressure deformable portions. The curved flow sensing conduits are solidly cantilever mounted from the inlet and outlet ports of the conduits, e.g. welded or brazed, so that the conduits can be oscillated in springlike fashion about axes which are located near the solidly mounted sections of the conduits. By further designing the mounted curved flow conduits so that they have resonant frequencies about the axes located near the solid mountings which are lower than the resonant frequencies about the axes which Coriolis forces act, a mechanical situation arises whereby the forces opposing generated Coriolis forces are predominantly linear spring forces. Oscillation of such a solidly mounted curved flow conduit while fluid is flowing through the flow conduit results in the generation of a Coriolis force couple. This Coriolis force couple is generated in two portions of the continuous flow conduit, to wit the portion where there is a velocity component of the fluid through the conduit directed toward the angular velocity vector, and the portion where there is a fluid velocity component directed away from the angular velocity vector. The Coriolis force couple opposed by linear spring forces twists or torques the curved conduit about an axis between the portions of the continuous flow conduit in which Coriolis forces are generated. The magnitude of the twisting or torquing is a function of the magnitudes of the generated Coriolis forces and the linear spring forces opposing the generated Coriolis forces.
The flow conduit in addition to being twisted by Coriolis forces is also being driven in oscillation. Accordingly, one of the portions of the continuous flow conduit on which the Coriolis forces are acting will be twisted so as to lead, in the direction in which the flow conduit is moving, and the other portion on which Coriolis forces are acting will be twisted so as to follow the first flow conduit section. The amount of time required for the respective twisted sections of the oscillating flow conduit to pass preselected points is a linear function of the mass flow rate of the fluid passing through the flow conduit. The relationship between the measured time and the mass flow rate passing through the flow conduit is only dependent on constants derived from the mechanics of the continuous flow conduit and its solid mounting. This relationship is not dependent on other variables which must be measured or controlled. Optical sensors are specifically described in U.S. Pat. No. Re. 31,450 and electromagnetic velocity sensors are specifically described in U.S. Pat. Nos. 4,422,338 and 4,491,025 for making the required time measurements from which mass flow rates can be determined.
A double flow conduit embodiment with sensors for making the necessary time measurements is specifically described in U.S. Pat. No. 4,491,025. The double flow conduit embodiment described in U.S. Pat. No. 4,491,025 provides a Coriolis mass flow rate instrument configuration which is operated in a tuning fork manner as described in U.S. Pat. No. Re. 31,450. The tuning fork operation contributes to minimizing effects of external vibration forces. Minimizing effects of external vibration forces is important because these forces can induce errors in the required time measurement. This embodiment also provides for accurate determinations of fluid mass flow rates without being limited by vibrational forces which can be transmitted through the support where the flow conduits are solidly mounted. The vibrational forces transmitted through the support which are of concern here are those caused by the oscillation of the flow conduits. As the mass of flow conduits increase, the forces transferred to the support by oscillating the conduits similarly increase. Because the flow conduits are configured in, and oscillated in, a tuning fork arrangement, the forces arising in the support are of equal magnitude. The forces are directed predominantly against each other and therefore cancel. As taught in U.S. Pat. No. Re. 31,450, it is possible to build single flow conduit Coriolis mass flow rate instruments without counterbalancing oscillating structures. Such single flow conduit Coriolis mass flow rate instruments, however, require supports which are massive in relation to the oscillated conduit so as not to be affected by the forces produced in association with oscillating the conduit. As a practical matter, in most industrial environments, instruments with flow conduits having inside diameters greater than about 1/16th inch are best constructed with tuning fork arrangements.
The support for an instrument can include multiple structures as taught in U.S. Pat. No. 4,491,025. In addition to welding or brazing the flow conduits to a first support structure, spacer bars, such as metal plates, can also be welded or brazed to adjacent portions of twin flow conduit embodiments at essentially equal distances from the first support structure. The combination of welding or brazing the flow conduits to the first support structure and to spacer bars results in an increase in the length of the flow conduit over which stress caused by oscillating the conduit is concentrated. This effective increase in the length of flow conduit decreases the strain experienced by the flow conduit and therefore provides a configuration which is less likely to produce cracks in oscillating flow conduits. The use of spacer bars also results in movement away from the first support structure of the axis about which the flow conduits are oscillated.